It is well recognized that a stagnation or decline in production of edible seafood, in particular, fish, by the marine fishing industry has occurred on a world wide basis. Since the world's population increases by approximately 100 million each year, maintenance of the present caloric content of the average diet will require production of an additional 19 million metric tons of seafood per year (United Nations Food and Agriculture Organization, The State of the World Fisheries and Aquaculture, Rome, Italy (1995)). In addition, fish products are becoming increasingly utilized in ways other than just food, for example, production of shells and pearls. To achieve this level of production, aquaculture (the cultivation of marine species) will have to double its production in the next 15 years, and wild populations of marine species must be restored.
Aquatic species includes marine teleost and elasmobranch fishes, fresh water teleost fish, euryhaline fish crustations, molusks and echinoderms. Marine teleost fish live in sea water with a high osmolality of about 1,000 mosm. Freshwater teleost fish normally live in water of less than 50 mosm. Euryhaline fish have the ability to acclimate to either of these environments. Ionic composition and osmolality of fish body fluids are maintained in these vastly different environments through gill, kidney and gastrointestinal tract epithelial cell function.
A major problem in aquaculture is development of methodology to rear marine teleost fish, such as cod, flounder and halibut, under freshwater hatchery conditions. To date, factors critical to the acclimation and survival of marine species to fresh water environments, and the control of these factors, have not been fully elucidated.
Attempts to develop such methodologies have also been complicated by problems with feeding the maturing larval forms of these fish. Development of cod, halibut or flounder species that could be reared in fresh water would be of great potential benefit in this regard. Under controlled fresh water conditions, developing forms of these fish could be raised in the absence of bacterial contamination normally present in seawater, and utilize new fresh water food sources that would potentially improve their survival.
The aquaculture industry utilizes the ability of young fish, e.g., salmon, (also called par) to be raised initially in fresh water and subsequently to be transferred for “growth out” in salt water pens as a means to produce large numbers of adult fish (young salmon tolerant to seawater are called smolt). Improvements in both the survival and health of fish undergoing the par-smolt transition would be very valuable for aquaculture growers.
Moreover, salmon that are kept in coastal marine “grow-out” pens during the winter are constantly at risk, since both winter storms, as well as exposure to extremely cold seawater, causes fish to freeze and die. These risks are further complicated by the fact that when adult salmon are adapted to salt water they do not readily readapt back to fresh water environment. Hence, lack of understanding of the means to readapt adult salmon from salt to fresh water results in the loss of salmon.
It is apparent, therefore, that there is an immediate need to develop methods of augmenting the survival of fish in fresh water and sea water, both in a natural environment and an aquacultural environment.